Internal combustion (IC) engines commonly controlair/fuel (A/F) ratio as a means by which to achieve a satisfactory combination of low fuel consumption, low exhaust emissions, good runability and high efficiency. In many cases, achieving these characteristics requires maintaining the A/F-ratio within relatively narrow limits by controlling operation of a fuel supply system that may employ, for example, a carburetor or a fuel injection system.
Engine speed is one parameter that is often controlled in connection with achieving good engine running characteristics. In some cases, control of engine speed is at least in part accomplished by skipping ignitions if the engine speed exceeds a cut out speed threshold to avoid over-revving the engine. When there is no spark applied above the speed limitation of the engine (i.e., the cut out speed threshold), the engine will generally charge better as the burned gases scavenge out. This will cause an increase in cylinder pressure (e.g., more than 100% in some cases) when the spark returns, which may cause damage to the cylinder or components therein and reduce the operating life of the engine. To address this concern, it may be possible to delay the ignition, i.e. reduce the ignition angle. Dependent upon size of delay and amount of time of applying the delay, delaying the engine ignition to reduce cylinder pressure could, however, cause problems with heat.
Conventionally, an ignition angle map (predetermined relation between ignition angle and rpm) would be used for controlling the ignition angle. To avoid a too high cylinder pressure when ignitions are again permitted after cut out, the ignition angle map would commonly include a drop in the ignition angle at a speed lower than the cut out speed. In some cases, the drop may be up to 5 degrees. This type of regulation may not be favorable as e.g. for some circumstances during normal operation the speed may not reach the cut out speed but will come close to the cut out speed and thus be within the reduced ignition angle range, possibly for an extended period of time. If the cut out speed is not reached, no sparks will be skipped and thus the speed will not be reduced by any logic (other than possibly as a result of the reduced ignition angle itself). A reduced ignition angle may then have the effect that there are still comparatively hot gases in the combustion chamber when the exhaust port is opened, and these will blow into the muffler, which may lead to severe heat problems. Meanwhile, increasing the amount of fuel may result in cylinder pressure reduction, but would cause higher fuel consumption and emission levels.